Fast startup: defer bootstrap to background, lazy feed load

- Node::open_with_bind no longer runs bootstrap (anchor connect, NAT
  probe, referrals). New run_bootstrap() method called from background
  task after UI is live.
- Background tasks (pull cycle, diff cycle, etc.) start after bootstrap
  completes, not during block_on.
- Feed no longer pre-loaded during welcome screen readiness check.
  Ready button enables immediately after get_node_info succeeds.
- Feed loads on tab switch, not during startup.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/app.js

@@ -3702,10 +3702,9 @@ async function init() {
             setupOverlay.classList.remove('hidden');
             setupName.focus();
         }
-        // Pre-load feed + messages from local DB (instant — no network needed)
-        await loadFeed(true).catch(() => {});
+        // Pre-load messages (lightweight) — feed loads when user switches to it
         loadMessages(true).catch(() => {});
-        // Mark ready button as clickable
+        // Mark ready button as clickable immediately — feed loads on tab switch
         if (readyBar) readyBar.style.width = '100%';
         if (readyBtn) {
             readyBtn.disabled = false;